Vertical cavity surface emitting lasers (VCSELs) commonly used in optical interconnects suffer from thermal lensing effects that lead to long turn-on delays and degradation of the error rates in interconnect systems.
In general, optical interconnects are used to link together computers and peripherals in networks requiring high speed data transport, low power consumption and immunity to electromagnetic interference. To achieve high bit rates, it has been necessary to use edge emitting laser diodes which present problems with packaging, power consumption, temperature sensitivity, and cost. Recently developed vertical cavity surface emitting lasers (VCSELs) provide a high performance alternative to edge emitters, simplifying the packaging problem, reducing power consumption and substantially reducing the cost of an optoelectronic transmitter module, and hence the cost of an entire optical interconnect link. The VCSELs employed to demonstrate optical interconnect performance have generally been of the proton-bombarded type, which exhibit strong thermal lensing effects that lead to large turn-on delays and large error rates in the presence of burst-mode data. While techniques that involve pre-biasing the lasers can alleviate the problem, these techniques necessarily add significant cost and complexity to the drive circuitry within the optical transmitter module. Constant duty-cycle coding schemes such as Manchester coding can also alleviate the turn-on delay problem, again at the cost of increased complexity and reduced flexibility.
In general, a digital optical interconnect converts an electrical bit stream at the transmitter to an optical bit stream, which propagates down an optical fiber or other optical waveguide until it reaches an optoelectronic receiver module where it is again converted to an electrical bit stream that faithfully replicates the bit stream at the transmitter end. Usually, the optical bit stream replicates the input electrical bit stream. However, in many data streams with arbitrary coding and burst transmission, there are long periods where no signal is sent at all. These long periods of inactivity allow the VCSELs in the transmitter module to cool off, which then results in a turn-on delay as long as a few microseconds when data transmission begins again. A delay of that length can lead to the loss of many bits at the beginning of the data stream. Thus there is a pressing need for a simple means to reduce the turn-on delay problems in VCSEL-based optical links.
Many complicated techniques to shape the input drive pulses to the source lasers for have been attempted. A scheme for pulse-shaping to minimize turn-on delay described in R. A. Nordin et al, Proc. 1993 ECTC, IEEE, pp. 795-801 (1993) is incorporated herein by reference.